1. Field of the Invention
The present invention relates to a magnetic bearing apparatus and to a magnetic bearing apparatus for reducing vibration caused by run-out in a radial direction of a rotor.
2. Description of the Related Art
A magnetic bearing is constituted by the arrangement of, for example, a plurality of coils (electromagnets) around both end portions of a rotor (rotary member). In an ordinary bearing, a rotor is pivotally supported by means of ball bearings or the like. However, in the magnetic bearing, a magnetic field generated by coils is applied to the rotor and an attractive force due to this magnetic field is balanced so that the rotor is supported (or levitated) in a non-contact manner in a constant position in space.
A run-out of the rotor (a shift in the radial direction from the constant position, i.e., a shift in the radial direction of the rotor) is detected by means of a radial direction sensor arranged in the vicinity of the magnetic bearing portion. In order to adjust the attractive force of the coils so that the run-out always falls within a constant range, a current of the coils is fed back and controlled.
In such a system where the rotor is supported by the magnetic bearing and rotated, there are some cases where a gravitational center (or inertia center) of the rotor and a rotary axis of the rotor are not identical to each other. When the rotor is rotated in such a condition, in the rotor, a run-out rotation in synchronism with a rotary cycle of the rotor caused by the misalignment between the gravitational center and the rotary axis is generated. In order to suppress the run-out rotation, the magnetic bearing generates a brake force in synchronism with the rpm of the rotor. Due to this cyclic brake force, the run-out in synchronism with the rpm of the rotor on the stator side where the magnetic bearing coils are arranged is generated in accordance with the law of action and reaction.
For instance, in the case where a turbo molecular pump carrying a magnetic bearing is used in an electronic microscope and so on, one of the more serious problems to be solved is how to control the vibration generated in the turbo molecular pump.
Attempts for suppressing the vibration caused by the above-described misalignment between the rotary axis of the rotor and the gravitational center thereof have been made. For instance, Japanese Patent Laid-open No. 259854/1995 discloses a magnetic bearing apparatus as such a magnetic bearing.
This magnetic bearing apparatus is shown in FIG. 11. A rotor 110 is pivotally supported at an air gap X0 from coils 107 and 108 by the balance of the attractive force of the coils. The vibration xcex94X of the rotor 110 is detected by means of a radial direction sensor 109 and a shift detector circuit 102. A magnetic bearing control circuit 101 feeds to power amplifiers 103 and 104 a signal of current to flow through the coils 107 and 108 for offsetting xcex94X.
In accordance with a signal of the magnetic bearing control circuit 011, the power amplifier 103 feeds a current I0+xcex94I to the coil 107 and the power amplifier 104 feeds a current I0xe2x88x92xcex94I to the coil 108. The rotor 110 is returned back to a constant position X0 by means of the attractive force of the coils 107 and 108 receiving this current feed. Here, I0 is a bias current to flow through the coils 107 and 108, and xcex94I is the shift component of coils by a positional control signal of the rotor generated in the magnetic bearing control circuit 101 when the air gap is generated by xcex94X.
The magnetic bearing apparatus disclosed in this Japanese Patent Laid-open No. 259854/1995 is to separately control xcex94I for a frequency (expressed as fr) equal to the rpm of the rotor 109 and the frequency components other than that out of power spectrum of the shift signal outputted from the radial direction sensor 109.
The explanation will be given with reference to formulae. In general, the attractive force F generated by the coils used in the magnetic bearing is given by the following formula.
xe2x80x83F=K{(I0+xcex94I)/(X0+xcex94X)}2xe2x80x83xe2x80x83(1)
Where, K is the constant determined by the number of turns or the shape of the coils.
A band-pass filter for passinc a signal of the frequency fr and a band-pass filter for passing signals other than the frequency fr are connected in parallel with each other within the magnetic bearing control circuit 101 The shift signal xcex94X of the rotor 110 is inputted into these filters and this signal is separated into the component having the frequency fr and the others (The components due to the run-out by the misalignment between the rotary center of the rotor 110 and the inertia center thereof is included in the signal of the frequency component fr).
For the shift signal xcex94X having the components other than the frequency fr obtained from the band-pass filters, the current of the coils are fed back and controlled in the same manner as in the conventional magnetic bearing apparatus.
The component xcex94X having the frequency fr obtained through the band-pass filters is controlled to become the value expressed by the following Formula (2). Then, the attractive force of the coils is kept constant as indicated by Formula (3).
xcex94I=(I0/X0)xcex94Xxe2x80x83xe2x80x83(2)
F=K(I0/X0)2xe2x80x83xe2x80x83(3)
As shown in Formula (3), since for the vibration having the frequency component fr, the attractive force for affecting the coils 107 and 108 is always kept constant, there is no vibration of the frequency fr in themagnetic bearing portion. Namely, the vibration (having the frequency fr) caused by the misalignment between the rotary axis and the inertia axis of the rotor 110 (in other words, it is safe to say that the gravitational center of the rotor 110 is not on the rotary axis) would not generate.
Note that a magnetic pole is provided in the central portion of the rotor supported at both ends by the magnetic bearings. The stator provided with coils (motor windings) is arranged around its periphery. A motor portion is formed in the rotor. In this case, when the rotor is vibrated in the radial direction, there is an unbalance in the magnetic force affecting the rotor by the magnetic field generated by the stator. This unbalance of the magnetic force is one of causes for generating the vibration upon the rotation of the rotor.
However, in the magnetic bearing apparatus disclosed in this Japanese Patent Application Laid-open, the vibration caused by the magnetic bearing per se may be suppressed but the vibration caused by the unbalance of the magnetic force in the above-described motor portion could not be suppressed although the existence thereof is clear.
Note that, brush-less DC motors carrying strong permanent magnets have been extensively used in accordance with the recent demands for miniaturization and high efficiency of the motors. In the case where the motor portions are formed by these motors, the vibration is likely to generate due to the magnetic unbalance concomitant with the phase of the permanent magnets in addition to the simple unbalance of the magnetic force caused by the difference in air gap intervals.
Further, there would be some cases where the rotor sticks to the stator due to the attractive force of the magnetic field so that the rotor could not be lifted by the magnetic bearings. In some cases, for this reason, the magnetic bearings must be enlarged more than necessary and the capacitance of the current amplifier must be increased.
Accordingly, in view of this point, an object of the present invention is to provide a magnetic bearing apparatus for reducing vibration in a bearing portion and a motor portion.
In order to attain the above-described object, according to the present invention, there is provided a magnetic bearing apparatus characterized by comprising a rotor, a motor portion provided in the rotor for rotating the rotor by a magnetic force, magnetic supporting coils for magnetically supporting the rotor in a predetermined position, magnetic force unbalance obtaining means for obtaining an unbalance of the magnetic force generated in the motor portion by the rotor run-out in the radial direction from the predetermined position and applied to the rotor, and magnetic support adjustment means for adjusting the magnetic force of the magnetic supporting coils so as to resist the unbalance of the magnetic force.
If the magnetic bearing apparatus is thus constructed, a force effective for resisting unbalance of the attractive force between the rotor and the stator caused by run-out of the position of the rotor in the radial direction is generated in the magnetic bearing to thereby make it possible to reduce the vibration of the stator.
Further, the predetermined position of the rotor may be the position obtained in the case where the rotor is rotated with the rotary axis of the rotor passing through the gravitational center of the rotor, i.e., when the rotor is rotated about the inertia center or the position obtained in the case where the magnetic force of the coils are corrected so that the magnetic force to be applied between the coils and the rotor is kept constant for one cycle through which the rotor is rotated for the variation of the air gap of the magnetic coils and the rotor and generated when the rotor is rotated about the intertia center.
The motor portion may be, for example, a brush-less DC motor. The brush-less motor is composed of a permanent magnet fixed to a rotor and a plurality of coils arranged around this permanent magnet. Then, the magnetic poles of these coils are switched over in order to generate a rotary magnetic field so that the permanent magnet of the rotor is caused to attract and follow the rotary magnetic field to thereby rotate the rotor.
When the position of the rotor is swung in the radial direction, an unbalance is generated in the attractive force between the permanent magnet and the coils. This is a cause for vibration of the motor portion. For this reason, the attractive force for offsetting the unbalance of this attractive force is generated in the coils of the magnetic bearing portion.
Further, it is possible to calculate the run-out of the rotor in the motor portion by detecting the run-out of the rotor by a radial direction sensor and calculate it from the geometric positional relation among the motor portion, the radial direction sensor, and the rotor, and the run-out value. If the run-out of the rotor in the motor portion is inferred in this way, it is possible to obtain the unbalance between the rotor and the stator in the motor portion from the angle of the magnetic field and the rotational angle of the magnetic poles of the rotor and the value of this run-out vibration through calculation or experimental values. Then it is possible to control the bearing force of the magnetic bearing so as to offset the unbalance of the magnetic force. In addition, the unbalance of the magnetic force may be kept in the form of a database of the angle of the magnetic field, the rotational angle of the magnetic poles and the run-out vibration of the rotor as variables in advance. Then, when the unbalance of the magnetic force is obtained from the run-out vibration of the rotor, it is possible to obtain the unbalance from the database by using the angle of the magnetic field, the rotational angle of the magnetic poles and the amount of run-out at this time.
Further, the unbalance of the attractive force due to the run-out of the rotor in the motor portion may be inferred from the value of the sensor directly mounted within the motor for detecting the magnetic flux between the rotor and the stator. The relationship of the unbalance of the magnetic force between the rotor, the stator and the value of the magnetic flux detected by the sensor, the angle of the magnetic field and the rotational angle of the magnetic poles are obtained through the magnetic field analyses or experiments.
Further, a sensor such as an acceleration sensor for obtaining the vibration generated by the rotation of the rotor is installed in, for example, the stator of the motor portion and is subjected to a mechanism for controlling the current of the coils of the magnetic bearing portion so that the vibration to be detected by the sensor upon the rotor rotation is reduced, whereby the vibration generated by the rotation of the rotor may be further reduced.